In the past, a computer system etc. has converted a digital image signal to an analog image signal and supplied the same to a display device by analog transmission. However, along with the spread of display devices for driving displays by using a digital signal, for example, liquid crystal displays, the method of digitally transferring a digital image signal without converting the same to an analog signal for preventing degradation of the quality of the image or reducing costs has been standardized. For example, the DDWG (Digital Display Working Group) is formulating a DVI (Digital Visual Interface, Version 1.0) specification. This DVI specification uses a TMDS (transition minimized differential signal) channel developed by Silicon Image Corp. of the U.S. to serially transfer a digital three-primary color image signal by the baseband for every color.
FIG. 4 is a conceptual view of a signal transfer system for transferring a digital image signal of content by using a channel of the DVI specification. A transmitter (for example a computer system or set top box) 20 for transmitting the image signal and a receiver (display device, television system, etc.) 50 for receiving the image signal and displaying the image are connected via channels 30 of the DVI specification. In a DVI specification channel, not only is a TMDS channel for transferring a digital image signal used as mentioned above, but also a bi-directional channel (I2C bus or the like) used for information transfer in a DDC (display data channel) specification relating to plug and play established by VESA (Video Electronics Standard Association) is provided.
Here, if a transmission controller 25 of the transmitter 20 detects that the receiver 50 has been connected, the transmission controller 25 communicates with a reception controller 53 of the receiver 50 according to the DDC protocol, controls an image signal generator 21 based on receiver information obtained from the reception controller 53, for example, information of a model name and resolution of the receiver 50, and generates a digital three-primary color image signal optimum for the receiver 50. The three-primary color image signal DG is supplied to a transmission processor 22 comprised by using a TMDS transmission circuit. The transmission processor 22 generates a baseband serial transfer signal SD from the supplied three-primary color image signal DG and supplies it to the receiver 50 from a connector 27 via the channel 30. The receiver 50 supplies the serial transfer signal SD supplied via a connector 51 to a reception processor 52. The reception processor 52 is comprised by using a TMDS reception circuit and generates the digital three-primary color image signal DS from the serial transfer signal SD and supplies it to an image display 54. For this reason, the image display 54 can optimize and display an image of the content based on the three-primary color image signal DS.
Since the content transferred via the channel 30 of TMDS is converted to the digital signal, a perfect copy can be prepared easily and limitlessly. This is a great advantage for the user, but a problem in terms of protecting the rights of the side providing a movie or other work. For this reason, a proposal referred to as “DVI-CP (Content Protection)” using encryption technology for copyright protection when transferring a digital signal of content by using a DVI specification channel has been made by Intel Corporation of the U.S.
FIG. 5 is a conceptual view of a signal transfer system for transferring a signal of content compatible with the DVI-CP specification. The image signal generator 21 of a transmitter 20a has a protection information detector 23 connected to it. This protection information detector 23 decides the digital three-primary color image signal DG to be transferred includes protection information indicating that the signal is one of content to be copyright protected. When this protection information detector 23 detects the protection information (when it is decided that the content is to be copyright protected), an encryption circuit 221 provided in a transmission processor 22a encrypts the three-primary color image signal DG and provides this encrypted signal, that is, the three-primary color image signal DGE, to the TMDS signal circuit 222, to generate the serial transfer signal SD.
Also, a reception processor 52a of a receiver 50a is comprised by using not only a TMDS reception circuit 521, but also a decryption circuit 522. The TMDS reception circuit 521 receives the serial transfer signal SD, generates a three-primary color image signal, and supplies it to the decryption circuit 522 for the decryption processing. Here, a transmission controller 25a of the transmitter 20a and a reception controller 53a of the receiver 50a perform processing for mutual authentication according to the DDC protocol and, when deciding that a correct party is connected, supply the encryption key DC used for the encryption of the three-primary color image signal DG to the decryption circuit 522 to enable the decryption circuit 522 to correctly decrypt the encrypted three-primary color image signal. The digital three-primary color image signal DG obtained at this decryption circuit 522 is supplied to the image display 54. Even if the three-primary color image signal DG is transferred encrypted, the image of content can be correctly optimized and displayed on an image display 3.
Also, since the serial transfer signal SD is generated based on the encrypted three-primary color image signal DGE, the unencrypted three-primary color image signal DS cannot be obtained just by receiving the serial transfer signal SD, so the content can be prevented from being digitally copied as it is.
If, however, encryption technology for copyright protection is added to this DVI specification and becomes standard after use of receivers 50 based on the above DVI specification starts, when a transmitter 20a based on the new specification encrypts and outputs a digital image signal, a prior DVI specification receiver 50 sometimes will not be able to display the image since it does not have the function of decrypting an encrypted digital image signal.
For example, as shown in FIG. 6, when outputting a digital image signal from a transmitter 20 based on the conventional DVI specification as a serial transfer signal SD, both a receiver 50 based on the conventional DVI specification or a receiver 50a based on the new specification including the encryption technology can display the image of the content. Also, when outputting a digital image signal from a transmitter 20a based on the new specification including the encryption technology as the serial transfer signal SD, a receiver 50a having the function of decrypting an encrypted digital image signal can display the image of the content. Since the receiver 50 does not have a decryption function however, sometimes the receiver 50 cannot be used for copying content without inviting degradation of the quality of the image etc., for example, it does not have the function of outputting the supplied digital three-primary color image signal or converting the three-primary color image signal to for example an NTSC system video signal for output and therefore cannot display the image of the content even if it is clear that copyright protection is possible.